A recent trend in tire design has been to extend features of the tread relatively far onto the sidewall of the tire. In many cases, the features extend to at least the tire equator. Tread features, for example, grooves, ribs, and blocks, are relatively deep or tall compared to the features typically formed on the sidewall, such as indicia or lettering.
In conventional molds, the tread ring is formed by taking an aluminum casting using a model. The sidewall shells are made by machining steel plate to have the desired molding elements. To form deep tread-like features in the sidewall shell poses difficulties, for example, in aligning the features with the tread rings and in machining deep or complex features in the sidewall shell surface.
The invention provides an improved tire mold and a method for manufacturing an improved tire mold that overcomes the difficulties in the art and provides additional advantages.
The invention is preferably applied to a two-piece segmented mold, the type which parts along or near an axial center line of the mold cavity and includes an upper half and a lower half, each half having tread sectors and a sidewall shell. Other molds may advantageously use the invention, for example, a one-piece segmented mold, in which the tread segments are not split along the axial center, but are-mounted to either the top or the bottom half of the mold, and mounted to pivot away upon opening the mold.
A tire mold according to the invention includes a plurality of tread sectors for forming a tread portion of a tire, a mold back for supporting the tread sectors and permitting movement between an open position and a closed position, two shell inserts for molding the sidewalls of a tire, and two side plates, each shell insert being mounted to a side plate, wherein the tread sectors and the shell inserts are castings.
According to another aspect of the invention, the shell inserts each have a circumferential mounting rib on a back surface for inserting into a mounting groove in the side plate. The mounting groove is wider in the radial direction than the mounting rib to accommodate differential expansions upon heating of the shell insert and the side plate to ensure proper mating of the tread sectors and the shell inserts during the tire molding process.
The shell inserts have vents extending from the molding surface (the front surface) through to the back surface, and grooves formed on the back surface to connect with vent passages in the side plate. The side plates each include passages on the mating surface that connect to interior passages that may be connected to a vacuum source.
The mold further includes two bead rings for molding the bead areas of the tire. The bead rings in the assembled mold are position on each half of the mold and overlap a portion of the shell inserts and will cover any gap present between the shell insert and the side plate in that area.
A method for manufacturing a mold in accordance with the invention includes the steps of providing a casting model having the tread and sidewall features of a tire, the casting model having parting zones between the tread and sidewall, casting an element from the model, the cast element accordingly having parting zones between the tread and sidewall portions, and separating the cast element into a plurality of tread sectors and opposing sidewall shells.
According to a preferred method, the casting elements are made in top and bottom halves divided along or near an axial centerline. A single casting model may be used, or upper and lower half models may be made, as is convenient. The two cast elements are then cut apart, the upper half element being separated into upper tread sectors and an upper sidewall shell for an upper half of the mold, and a lower cast element being separated to form lower tread sectors and a lower sidewall shell for a bottom half of the mold.
According to an alternative embodiment, a casting model is made without a parting zone between the tread and sidewall, and for each of the upper and lower half, at least two cast elements are formed, a first cast element separated into a plurality of tread sectors and a second cast element separated into at least one sidewall shell.
An advantage of casting the molding parts, the tread sectors and sidewall shells from a single casting model is that deeper and more complex features can be formed in the sidewall shells, and the features extending from the tread onto the sidewall can be better aligned and better conformed in size, shape and appearance.